Martian impacts and Phobos' grooves.

Martian impacts and Phobos' grooves

Scientists continue to propose new explanations for the unusual surface grooves on the Martian moon Phobos, including debris tossed out by meteorite impacts, the release of trapped gas deposits, and cracks associated with the formation of the crater named Stickney (SN: 11/4/89, p.301). Now two scientists suggest the grooves on Phobos resulted from debris ejected when other Phobos-sized objects collided with Mars.

According to Peter H. Schultz and David A. Crawford of Brown University in Providence, R.I., observations of both Phobos and Mars, primarily by the two U.S. Viking orbiters, appear "consistent with separate epochs of collisions by linear chains of Mars-orbiting debris."

A few years ago, Schultz cited several examples of what he said looked like scars on Mars caused by an unusually large number of objects that struck its surface at angles of less than 15 degrees. Such shallow impacts, he and Crawford now maintain, provide independent support for the idea of several short-lived, close-in Martian satellites. The grazing impacts are too large for small ring-particles to make, but Schultz says laboratory studies show that oblique impacts onto the sruface of Mars as the orbits of bigger objects decayed could have produced expanding vapor clouds that would have injected significant amounts of material into orbit. On Phobos, the authors report, the grooves caused by running into such material would be limited to parallel or nearly parallel arcs less than 180 degrees long, and indeed show a similarity to the grooves in the spacecraft photos of the Martian moon.